Production of sintered moldings

ABSTRACT

Sintered moldings are produced by molding a mixture of a sinterable ceramic or metallic powder and polyoxymethylene or a copolymer containing a majority of oxymethylene units as binder to give a compact, removing the binder by treatment with a gaseous acid, and sintering the product, which comprises removing the binder using an acid which is solid at room temperature and sublimes or melts and evaporates at elevated temperatures.

The present invention relates to an improved process for the productionof sintered moldings by molding a mixture of a sinterable ceramic ormetallic powder and polyoxymethylene or a copolymer containing amajority of oxymethylene units as binder to give a compact, removing thebinder by treatment with a gaseous acid, and sintering the product.

EP-A-413 231 describes a process for the production of inorganicsintered moldings in which compacts of sinterable powders andpolyoxymethylene as binder are injection molded or extruded. The binderis removed from these compacts by treatment with a gaseous acid orgaseous boron trifluoride, preferably in a stream of inert carrier gas,and the resultant parts are sintered.

The earlier application P 42354293 discloses a further-developed processfor the production of sintered moldings in which the binderpolyoxymethylene is removed under reduced pressure and thus withoutusing a carrier gas.

A common feature of both processes is the type of acids to be used forremoving the binder, namely acids which are gaseous at room temperature,for example hydrogen halides and hydrogen sulfide, or liquid at roomtemperature, such as nitric acid, sulfuric acid, formic acid and aceticacid. However, these acids remain in the gas phase and corrode allapparatus parts which come into contact with the gas phase, or formliquid films which are likewise corrosive. In addition, disposal ofthese gases is not straightforward.

It is an object of the present invention to overcome these problems.

We have found that this object is achieved by a process for theproduction of sintered moldings by molding a mixture of a sinterableceramic or metallic powder and polyoxymethylene or a copolymercontaining a majority of oxymethylene units as binder to give a compact,removing the binder by treatment with a gaseous acid, and sintering theproduct, which comprises removing the binder using an acid which issolid at room temperature and sublimes or melts and evaporates atelevated temperatures.

Suitable sinterable powders for the novel process are oxidic, ceramicpowders, such as Al₂ O₃ Y₂ O₃, SiO₂, ZrO₂, TiO₂, Al₂ TiO₅ or YBa₂ Cu₃o_(7-x) as ceramic superconductor. Also suitable are oxide-free ceramicpowders, such as Si₃ N₄, SiC, BN, B₄ C, AlN, TiC, TiN, TaC and WC.

Examples of suitable metal powders are Fe, Al, Cu, Nb, Ti, Mn, V, Ni,Cr, Co, Mo, W and Si powders. The metal powders can also be employed inthe form of alloys, for example as intermetallic phases such as TiAl,Ti₃ Al and Ni₃ Al. Graphite and carbon black are also suitable. It is ofcourse also possible to use mixtures of said materials.

The particle size of the powders is generally from 0.005 to 100 μm,preferably from 0.1 to 30 μm, particularly preferably from 0.2 to 10 μm.

The binders to be employed according to the invention comprisepolyoxymethylene, advantageously having a molecular weight of from10,000 to 500,000. In addition to homopolymers of formaldehyde ortrioxane, copolymers of trioxane with, for example, cyclic ethers, suchas ethylene oxide and 1,3-dioxolane, or formals, such as butanediolformal, are also suitable, the amounts of the comonomers generally beingfrom 1 to 4% by weight of the polymers.

The materials to be molded usually contain from 40 to 70% by volume ofthe sinterable powder in addition to the binder. Furthermore, inorganicfibers or whiskers of, for example, Al₂ O₃, SiC, Si₃ N₄ or C can beadded to the materials, which may also contain auxiliaries, such asdispersants, lubricants, such as polyethylene glycol or stearic acid, orfurther thermoplastic binders, such polyethylene, polymethylmethacrylate or polyethylene oxide.

The amount of auxiliaries is Generally from 0.1 to 12% by weight of thetotal material.

After all the components have been mixed, for example in a compounder orextruder, the materials are molded, for example by injection molding inconventional screw or plunger injection-molding machines, at from 160°to 200° C. and at from 500 to 2000 bar.

The resultant compacts are treated with acids, which degrade the binderpolyoxymethylene to give gaseous products, predominantly formaldehyde.The gaseous degradation products are usually removed from the reactionzone.

Suitable acids are those which are solid at room temperature and sublimeor melt and evaporate at elevated temperature, preferably those having asublimation or melting point of between 25° and 200° C.

Particular preference is given to anhydrous oxalic acid or oxalic aciddihydrate. Glyoxalic acid is also suitable, as are benzenesulfonic acid,naphthalene-sulfonic acids and maleic acid, and mixtures thereof. Duringbinder removal, these can be employed either alone or together with acarrier gas, such as air, nitrogen or a noble gas.

At the binder-removal temperature, the acids to be used according to theinvention are first converted into the gas phase, where they act on thebinder and sublime or solidify after cooling on the walls of thebinder-removal apparatus. In a subsequent binder-removal step, they areagain converted into the gas phase, ie. virtually no acid escapes fromthe apparatus. This is a particularly preferred embodiment of the novelprocess.

In order to simplify metering, it may be expedient to employ theabovementioned acids as solutions in polar solvents, preferably havingboiling points of below 200° C. Suitable solvents are, in particular,acetone, dioxane, ethanol and acetonitrile.

The temperature during binder removal is generally from 100° to 160° C.,preferably below the softening point of the binder.

The binder removal can be carried out under reduced pressure orpreferably at atmospheric pressure, in which case a carrier gas, inparticular nitrogen, is also used. There is no need for the carrier gaswhen binder removal is carried out under reduced pressure.

The novel process has the advantage that, due to their solid form atroom temperature, the acids to be used are easy to transport and meterand require no special safety precautions.

A further advantage is that the binder can easily be removed fromcompacts made from oxidation-sensitive sinter materials such as WC/Coand Cu.

EXAMPLES Example 1

Powders 1 to 8 shown in the table, with mean particle sizes PS, werecompounded in the stated amounts with a polyoxymethylene copolymer madefrom trioxane and 2% by weight of butanediol formal having a meanmolecular weight of 150,000 and with 2% by weight of polyethylene (Nos.1 to 3) or with 2% by weight of polyethylene glycol (Nos. 4 to 8) havinga molecular weight of 800 as lubricant, and were injection-molded togive rods measuring 5×7×65 mm³.

In accordance with the invention, these rods were treated at 135° C.under atmospheric pressure with 150 g of anhydrous oxalic acid and 500l/h of nitrogen as carrier gas for 6 hours in a 50 l oven for binderdegradation. The weight loss of anhydrous oxalic acid was 85 g.

The resultant rods showed no oxidation of the surfaces, were crack-freeand had the densities D_(s) shown in the table after sintering at thetemperatures T_(s).

Example 2

The Al₂ O₃ rods produced as described in Example 1 (No. 4) were treatedin accordance with the invention at 130° C. and 50 mbar with 40 g ofoxalic acid dihydrate for 6 hours in a 3 l oven for binder degradation.After binder removal, 27 g of oxalic acid dihydrate had sublimed out.

The weight loss of the resultant rods was 19.2%. After sintering at1600° C. in air, they had a density of 3.89 g/cm³.

Example 3

The Al₂ O₃ rods produced as described in Example 1 (No. 4) were treatedin accordance with the invention at 135° C. and 100 mbar with 20 g ofbenzenesulfonic acid and 200 l/h of nitrogen as carrier gas for 8 hoursfor binder degradation. After binder removal, all the benzenesulfonicacid had evaporated.

The weight loss of the resultant rods was 19.3%. After sintering at1600° C. in air, the density was 3.90 g/cm³.

                                      TABLE                                       __________________________________________________________________________               PS POM BDF                                                                              PE PEG                                                                              WL.sub.th                                                                        WL.sub.br                                                                         T.sub.s                                                                          D.sub.s                                                                            D.sub.th                            No.                                                                              Powder  [μm]                                                                          [g] [g]                                                                              [g]                                                                              [g]                                                                              [%]                                                                              [%] [°C.]                                                                     [g/cm.sup.3 ]                                                                      [g/cm.sup.3 ]                       __________________________________________________________________________    1  2000 g                                                                            Fe  3.5                                                                              198 4  44 -- 9.0                                                                              9.0 1280                                                                             7.64 7.86                                2  1960 g                                                                            Fe  3.5                                                                              198 4  44 -- 9.0                                                                              8.9 1280                                                                             7.65 7.90                                   40 g                                                                              Ni  7.0                                                                3  4500 g                                                                            WC  0.8                                                                              311 6  106                                                                              -- 5.9                                                                              5.9 1500                                                                             14.0 14.5                                   500 g                                                                             Co  2.5                                                                4  1000 g                                                                            Al.sub.2 O.sub.3                                                                  0.7                                                                              239 5  -- 25 19.3                                                                             19.2                                                                              1600                                                                             3.90 3.97                                5  1000 g                                                                            ZrO.sub.2                                                                         0.4                                                                              195 4  -- 24 16.3                                                                             16.3                                                                              1500                                                                             6.05 6.08                                6  976 g                                                                             SiC 0.4                                                                              394 8  -- 28 28.1                                                                             20.0                                                                              2150                                                                             3.10 3.22                                   4 g B   0.8                                                                   20 g                                                                              C   0.025                                                              7  930 g                                                                             Si.sub.3 N.sub.4                                                                  0.7                                                                              325 7  -- 27 24.4                                                                             24.5                                                                              1850                                                                             3.19 3.25                                   50 g                                                                              Y.sub.2 O.sub.3                                                                   0.4                                                                   20 g                                                                              Al.sub.2 O.sub.3                                                                  0.7                                                                8  840 g                                                                             Si  2.0                                                                              426 9  -- 29 29.7                                                                             29.7                                                                              1750                                                                             3.23 3.33                                   80 g                                                                              Y.sub.2 O.sub.3                                                                   0.4                                                                   80 g                                                                              Al.sub.2 O.sub.3                                                                  0.7                                                                __________________________________________________________________________     PS: particle size                                                             POM: polyoxymethylene                                                         BDF: butanediol formal                                                        PE : polyethylene                                                             PEG: polyethylene glycol                                                      WL.sub.th : theoretical weight loss on binder removal                         WL.sub.br : weight loss on binder removal                                     T.sub.2 : sintering temperature                                               D.sub.s : density after sintering                                             D.sub.th : theoretical density after sintering                           

We claim:
 1. A process for the production of sintered moldings whichcomprises: molding a mixture of a sinterable ceramic or metallic powderand polyoxymethylene or a copolymer containing a majority ofoxymethylene units as binder to give a compact, removing the binder bytreatment with a gaseous acid,a nd sintering the molding wherein theacid is solid at room temperature and sublimes or melts and evaporateswhen heated.
 2. The process of claim 1, wherein the binder is removedusing acids having a sublimation or melting point of between 25 and 200°C.
 3. The process of claim 2, wherein the binder is removed usinganhydrous oxalic acid or oxalic acid dihydrate.
 4. The process of claim2, wherein the binder is removed at atmospheric pressure or underreduced pressure.
 5. The process of claim 1, wherein the binder isremoved using anhydrous oxalic acid or oxalic acid dihydrate.
 6. Theprocess of claim 5, wherein the binder is removed at atmosphericpressure or under reduced pressure.
 7. The process of claims 1, whereinthe binder is removed at atmospheric pressure or under reduced pressure.